The present invention relates generally to optical systems and, more particularly, to a method and Raman pump unit for reducing noise transfer from a pump to a main optical signal travelling on a main optical path.
Stimulated Brillouin Scattering (SBS) is a serious impairment in fiber optic communication systems. SBS is a nonlinear optical process that can occur in an optical fiber when a narrow spectral linewidth laser source is used to transmit optical signals. Among various nonlinear effects, the impact of SBS is by far the most severe because of the relatively low optical power at which it can occur. SBS manifest itself through the generation of a backward-propagating wave that carries a significant portion of the input signal energy (the Brillouin gain) once a certain optical power threshold (the Brillouin threshold) is reached. This process negatively impacts optical system performance by reducing the optical power received at a receiver downstream. In order to launch higher input power into the fiber, it is desirable to raise the Brillouin threshold, which can be achieved using a xe2x80x9csource ditheringxe2x80x9d technique at a transmitter site. Basically, this technique consists of varying the intensity of a main optical signal being transmitted with a small-amplitude dither signal having frequency content in the MHz range. The net effect of this technique is to broaden the spectral linewidth of the source signal, which allows the Brillouin gain to be highly reduced and as a result eliminates the detrimental effects of SBS on optical systems.
Fiber optic communication systems are also prone to other impairments, such as signal attenuation, which are due to lengthy transmission spans. One way to combat this type of impairment, which may be used in conjunction with source dithering, is the constructive usage of the so-called Raman effect in optical fibers. Specifically, it is possible to use intrinsic properties of silica (glass) fibers to obtain signal amplification. This means that the transmission fiber can be used as a medium for amplification, allowing the attenuation of data signals transmitted over the fiber to be combated within the fiber itself. An amplifier working on the basis of this principle is commonly known as a Raman amplifier.
The physical property behind a Raman amplifier is stimulated Raman scattering (SRS, not to be confused with SBS). This occurs when a pump signal of a sufficiently high pump power is launched at a shorter wavelength (higher optical frequency) than the main optical signal to be amplified. Amplification occurs when a photon from the pump signal gives up its energy to create a new photon at the signal frequency, plus some residual energy, which is absorbed as vibrational energy. The resulting amount of gain (known as xe2x80x9cRaman gainxe2x80x9d) thus depends strongly on the pump power and the frequency offset between the pump signal and the main optical signal. As is well known in the art, Raman amplification potentially can be achieved in every region of the transmission window of a conventional optical transmission fiber, as long as a powerful pump source is available at the required wavelength.
However, it will be noted that the pump signal used in Raman amplification is itself not immune to SBS. Therefore, techniques similar to source dithering have been applied to the reduction of SBS in Raman amplifiers. This technique is aptly named xe2x80x9cpump ditheringxe2x80x9d, in analogy to xe2x80x9csource ditheringxe2x80x9d used for the main optical signal. Just as source dithering requires the source signal to be modulated by a low-amplitude dither signal having frequency content in the MHz range, pump dithering requires the laser drive current of the pump source to be modulated by a low-amplitude dither signal having frequency content in the MHz range. Not surprisingly, this has the effect of broadening the spectral linewidth of the pump signal, which has been found to avoid the detrimental effects of SBS arising from use of a pump signal.
The parameters of the low-amplitude dither signal used in pump dithering are typically set such that the modulated pump signal continues to perform well in the presence of SBS. Because one of the drawbacks of Raman amplification is the need for high pump powers in order to provide reasonable gain, it turns out that the xe2x80x9clow-amplitudexe2x80x9d dither signal used to modulate the pump signal will have a rather non-negligible power. The ultra fast response time of the Raman amplification process means that any time varying fluctuations in pump signal amplitude, phase or frequency will get transferred almost instantaneously to the main optical signal, which causes the main optical signal to feel the effects of the dithering in the pump signal. These effects are especially noticeable in the case where the pump signal and the main optical signal travel in the same direction along an optical transmission fiber (referred to as a xe2x80x9cco-pumpingxe2x80x9d or xe2x80x9cforward pumpingxe2x80x9d scenario).
The variations in the amplitude of the main optical signal constitute noise, which could potentially lead to corruption of the data carried by the main optical signal. Moreover, when multiple spans are involved, each utilizing Raman Amplification, the effect of source dithering on signal quality is accumulated as amplitude modulation gets transferred from each successive pump source to the main optical signal.
Against this background, it should be apparent that the need exists to provide a solution by which pump dithering for countering SBS in a Raman amplifier can continue to be practiced, while only minimally affecting the integrity of the optical signal being amplified.
According a first broad aspect, the invention provides a Raman pump unit for connection to a main optical path. The Raman pump unit includes a signal generator capable of generating a plurality of at least partially complementary dither signals, a plurality of modulators connected to the signal generator, each modulator capable of receiving a corresponding pump driver signal and modulating the corresponding pump driver signal in accordance with a corresponding one of the dither signals, a plurality of pumps connected to the plurality of modulators, each pump capable of receiving a corresponding modulated pump driver signal from a corresponding one of the modulators and producing a corresponding optical pump signal therefrom and an optical coupler connected to the plurality of pumps, the optical coupler capable of introducing a combination of the optical pump signals into the main optical path.
According to another broad aspect, the invention provides a method of modifying a main optical signal travelling on a main optical path. The method includes a step of generating a plurality of dither signals, each dither signal having a temporal variability. The method further includes the step of modulating a plurality of pump driver signals with the plurality of dither signals, respectively, to produce a respective plurality of modulated pump driver signals. The method also includes the step of producing a plurality of optical pump signals from the plurality of modulated pump driver signals, each optical pump signal having a respective spectral width. Finally, a combination of the optical pump signals are introduced into the main optical path. The dither signals are designed selected such that the combination of the optical pump signals has a greater spectral width than any of the optical pump signals in the absence of dithered and a lesser temporal variability than any of the individually dithered optical pump signals.
According to a third broad aspect, the present invention provides a signal embodied in a transmission medium. The signal includes a main optical signal having spectral content centered about a main optical wavelength and an optical pump signal having spectral content in a region occupying at least one second wavelength shorter than the first wavelength. The optical pump signal is characterized by a plurality of individual optical component signals, each individual optical component signal having spectral width and a time-varying intensity brought on by dithering a corresponding pump driver signal with a corresponding dither signal, the pump signal having a spectral width that is greater than the spectral width of any of the individual optical component signals in the absence of dither and a temporal variability that is less than the temporal variability of any of the individual optical component signals.
The present invention affords several advantages. Firstly, the use of pump dithering at the pump wavelength (or wavelengths) increases the spectral width of the combination of optical pump signals being injected into the main optical path. This spectral widening helps to fight the deleterious effects of SBS. Meanwhile, the signal gain (at the wavelength of the main optical signal) is the combination of a plurality of dithered waveforms that are at least partially complementary in time. This combination will cause in a xe2x80x9cself-averagingxe2x80x9d effect, which causes less pump noise to be transferred to the main optical signal.
As can be appreciated, the present invention finds application in various types of Raman amplification, such as in Distributed Raman Amplification (where the transmission fiber is used as the amplification medium) or in Discrete Raman Amplification where gain is provided to various spectral regions of the transmission window within an amplifier module.
These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings.